An outer ignition motor (EC motor) is an intensity motor where an inner working liquid is warmed by burning of an outside source, through the motor wall or an intensity exchanger. The fluid then causes motion and usable work by expanding and acting on the engine’s mechanism.

The fluid is then cooled, compressed, and used again (closed cycle), or it is dumped and cool fluid is pulled in (open cycle air engine), which is less common.

The process of burning fuel with an oxidizer to generate heat is referred to as combustion. Similar or even identical engines may utilize heat from nuclear, solar, geothermal, or exothermic reactions that do not involve combustion as a source of heat; but are now considered external thermal engines instead of external combustion engines.

The functioning liquid can be a gas as in a Stirling motor, or steam as in a steam motor or a natural fluid, for example, n-pentane in a Natural Rankine cycle. Any type of fluid can be used; Despite the fact that single-phase liquids are occasionally used, gas is by far the most common type. The fluid in the steam engine transitions between liquid and gas phases.

Instead of carrying an oxidizer like in a rocket, air-breathing combustion engines use oxygen in the atmosphere to oxidize (or “burn”) the fuel. This ought to have a better specific impulse in theory than rocket engines.

The engine that breathes air moves a constant stream of air through it. After being compressed, this air is mixed with fuel, ignited, and released as exhaust gas. Exhaust gas provides direct thrust in reaction engines, where the majority of the combustion energy (heat) exits the engine.